The present invention relates to ceramic matrix composites, and to methods for the manufacture thereof. More particularly, the invention relates to an improved method for manufacturing preforms for ceramic matrix composites.
Ceramic matrix composites are well known. Typically they comprise a ceramic matrix material such as a glass, a glass-ceramic, or a crystalline ceramic material forming a matrix wherein refractory inorganic fibers are disposed as a reinforcing phase. A number of U.S. patents show the structure and manufacture of such composites, including, for example, U.S. Pat. Nos. 4,314,852, 4,588,699 and 4,615,987.
In conventional practice, composites incorporating ceramic matrix materials are manufactured from powders of the selected matrix or a matrix precursor, with the fibers being combined with the powders and consolidated to dense composite materials by the application of heat and pressure. The fibers can be short chopped fibers or whiskers, but are more frequently introduced as a continuous fiber strand, consisting of one or more often a multiplicity of fibers in the form of a continuous yarn or tow. To provide a homogeneous distribution of the fibers in the matrix, the powders are generally first dispersed in a liquid vehicle. The resulting suspension is then applied to the fibers by an impregnation or coating process or, in the case of short fibers, by mixing the fibers into the suspension.
After the fibers have been coated or impregnated with the liquid suspension, they are typically laid up into a preform or prepreg comprising the fibers and the suspension containing the powdered material. Frequently, the fiber is provided in the form of a continuous fiber tow, and the prepreg is formed by winding the suspension-impregnated tow onto a rotating support such as a drum.
Using conventional solvent-based liquids as vehicles for the powdered ceramics, the suspensions remain fluid during the fiber impregnation and collection steps, such that desirable integration of the tows being collected occurs. Thus the collected mass of tow becomes a unitary mat. Following collection, the collected material is dried, i.e., the liquid solvents and carriers constituting the vehicle are removed by evaporation, such that an integral dry preform or prepreg which can be conveniently handled is provided.
Referring to specific ceramic suspensions known to the art, U.S. Pat. No. 4,412,854 discloses the manufacture of a fiber-reinforced composite preform from a suspension of a glass powder in a vehicle comprising a thermoplastic polymeric binder, typically an acrylic polymer binder. The binder is dispersed or dissolved in a carrier liquid and powdered matrix material is added. The resulting liquid suspension is combined with a woven or non-woven fiber reinforcement which is then dried and optionally reshaped prior to consolidation.
U.S. Pat. No. 4,588,699 illustrates the use of an aqueous system wherein latex is employed as a binder, and also a solvent-based system wherein the matrix is simply dispersed in a liquid carrier of propanol.
More complex suspension systems are disclosed in U.S. Pat. No. 4,585,500, wherein a synthetic resin is dispersed in a solvent for the resin prior to the introduction of the powdered matrix material. The system further includes components such as paraffin wax, a plasticizer and a surfactant to act as dispersion and stabilization aids and to impart flexibility to the resulting preform after drying.
A recently developed prepreg fabrication procedure, used with particular advantage for the fabrication of ceramic matrix composites, involves the use of suspensions of powdered ceramic matrix materials in thermoplastic vehicles. These are organic vehicle formulations which are room temperature solids, yet which will melt and flow as liquids at moderately elevated temperatures.
In the preparation of prepreg tows from these materials, suspensions of ceramic matrix powder in the heated, liquified binders are first prepared, and then reinforcing fiber tow is impregnated with the liquified suspensions, typically by immersion of the tow therein. The infiltrated tow is then generally withdrawn from the suspension, quickly cooled to a non-tacky state, and then collected on a revolving drum or spool.
Due to the speed with which prepreg tow comprising only thermoplastic binders will solidify, it has proven difficult to form prepreg mat during the tow collection process. Methods have been developed to form collimated tape from prepreg tow of this type, by reheating and fusing multiple yarn ends into a continuous fused ribbon. However, up to the date of the present invention, hot tow collection schemes and other attempts to continuously produce thick prepreg mat had not been successful.
It is therefore a principal object of the present invention to provide a method for making prepreg mat comprising reinforcing fibers and ceramic matrix material in a thermoplastic binder which provides improved structural homogeneity in the prepreg mat material.
It is a further object of the invention to provide a method for making prepreg mat of homogeneous structure which retains good flexibility and excellent thermal reshaping characteristics, due to the presence of thermoplastic binders therein.
It is a further object of the invention to provide a method for making prepreg mat comprising thermoplastic binder wherein the configuration of the mat can be closely controlled.
It is a further object of the invention to provide apparatus for the fabrication of dense, homogeneous preforms for ceramic matrix composite articles from prepreg fiber tow or other stranded fiber.
Other objects and advantages of the invention will become apparent from the following description thereof.